BIOLOGISTS believe that love is fundamentally a biological rather than a cultural construct. That is because the capacity for love is found in all human cultures and similar behaviour is found in some other animals, such as prairie voles. In humans the purpose of all the cravings, craziness and desire is to focus attention on the raising of offspring. Children demand an unusual amount of nurturing, and two parents are better than one. Love is a signal that both partners are committed, and makes it more likely that this commitment will continue as long as is necessary for children to reach independence. But what does science have to say about the notion of love at first sight?